For purifying exhaust gas containing, for example, CO, HC, and NO, precious metal elements (Pt, Rh, Pd, and Ir) exhibit a high performance. Therefore, it is preferable to employ the above-mentioned precious metal elements to the purification catalyst for exhaust gas. These precious metals are generally supported by Al2O3 which is a support having a high surface-to-weight ratio. On the other hand, composite oxides (for example, a perovskite-like oxide) made by combining various elements have extremely varied properties. Therefore, it is preferable for a purification catalyst for exhaust gas to employ the above-mentioned composite oxides. Moreover, when the precious metal is supported by the composite oxides, the properties of the precious metal are significantly changed. From this viewpoint, a preferable performance for purifying exhaust gas can be obtained in the purification catalyst for exhaust gas in which a precious metal is supported by a composite oxide.
Various catalysts mentioned above are now developed, and for example, a technique in which a coalescence rate of the precious metal can be reduced by setting a perovskite-like composite oxide to be a support, judging from deterioration of the precious metal with reduction of active sites by coagulation of the precious metal, is proposed (see the claims of Japanese Unexamined Application Publication No. 5-86259). Moreover, another technique in which reduction of PdO can be reduced by using a perovskite-like composite oxide in which the A site is defective, judging from reducing PdO which is an activated species in a NO reduction reaction, whereby the PdO changes to Pd which is low-active Pd, when the precious metal is Pd, is proposed (see the claims of reacting of Japanese Unexamined Application Publication No. 2003-175337).
Conventional purification catalysts for exhaust gas exhibit sufficient performance for reducing CO, HC, and NOx contained in exhaust gas, in a running of vehicle, particularly during a running at high temperatures (not less than 400° C.). However, the conventional catalysts cannot exhibit sufficient performance for reducing CO, HC, and NOx, in a vehicle at the starting or idling thereof at low temperatures (not more than 400° C.).
As mentioned above, the reason that sufficient performance for purifying the exhaust gas cannot be obtained in the running at low temperature is as follows. That is, in the conventional purification catalyst for exhaust gas, precious metal, for example, Pt, Rh, or Pd, is supported on Al2O3 having a high surface-to-weight ratio. Due to the high surface-to-weight ratio of the Al2O3, the precious metal is advantageously supported in a highly dispersed condition. However, Al2O3 is a stable compound, and does not mutually affect a supported precious metal, whereby activity of the precious metal is not improved. Accordingly, sufficient performance during the running at low temperature cannot be obtained.
Moreover, in the running of a vehicle, it is preferable for Pd to exist in a condition of PdO which is highly reactive. However, even if Pd supported on the Al2O3 initially exists in a condition of PdO, the Pd is reduced to be a metal condition at high temperatures, whereby the activity is significantly reduced.